Pim-1 is an oncogene-encoded serine/threonine kinase primarily expressed in hematopoietic and germ cell lines. The Pim-1 oncogene was originally identified as a preferred site for proviral integration of the slow transforming Maloney murine Leukemia Virus (MuLV)-induced in lymphoblastic T-cells and is associated with multiple cellular functions such as proliferation, survival, differentiation, apoptosis and tumorigenesis (Wang et al., J. Vet. Sci. 2: 167-179 (2001)). Direct evidence for the oncogenic potential of the Pim-1 gene comes from the study of transgenic mice in which overexpression of Pim-1 produces a low but spontaneous rate of tumor incidence (Domen et al., Leukemia 7 (Suppl. 2):S108-112 (1993)). These mice are highly susceptible to chemical carcinogens, X-ray radiation and MuLV-induced lymphomagnesis. In most cases, this correlated with the upregulation of C- or N-myc genes suggesting synergism between the Pim-1 and myc genes in the development of lymphomas (Breuer et al., Cancer Res. 51: 958-963 (1991); van Lohuizen et al., Cell 56: 673-682 (1989)). Pim-1 knockout mice did not show any obvious phenotype suggesting in vivo functional redundancy of this highly conserved oncogene (Domen et al., J. Exp. Med. 178: 1665-1673 (1993)).
Since the initial report of the cloning of mouse Pim-1 gene (Selten et al., Cell, 46: 603-611 (1986)), Pim-1 has been cloned from human, rat, bovine and zebrafish cDNA libraries (Wang et al., J. Vet. Sci. 2: 167-179 (2001)). In humans, the Pim-1 gene is expressed mainly in the developing fetal liver and spleen (Amson et al., Proc. Natl. Acad. Sci. U.S.A. 86: 8857-8861 (1989)) and in hematopoietic malignancies (Nagarajan et al., Proc. Natl. Acad. Sci. U.S.A. 83: 2556-2560 (1986); Meeker et al., Oncogene Res. 1: 87-101 (1987)). Two homologues of the Pim-1 gene, pim-2 (Allen et al., Oncogene 15: 1133-1141 (1997); van der Lugt et al., Embo J. 14: 2536-2544 (1995)) and pim-3/kid-1 (Feldman et al., J. Biol. Chem. 273: 16535-16543 (1998)) have also been identified.
The expression of Pim-1 is tightly regulated and is induced by cytokines, mitogens and hormones: IL-2, IL-3, IL-5, IL-6, IL-7, IL-9, IL-12 and IL-15, granulocyte-macrophage colony-stimulating factor (GM-CSF), erythropoietin, ConA, PMA, interferon-γ and prolactin (Wang et al., J. Vet. Sci. 2: 167-179 (2001)). The JAK/STAT pathway may be one of several signaling pathways that mediate Pim-1 expression (Nagata et al., Leukemia 11(Suppl 3): 435-438 (1997); Sakai and Kraft, J. Biol. Chem. 272: 12350-12358 (1997); O'Farrell et al., Blood 87: 3655-3668 (1996); Kumenacker et al., J. Neuroimmunol. 113: 249-259 (2001)). However, results from a study by Krishnan and colleagues (Krishnan et al., Endocrine 20: 123-130 (2003)) do not support a role for the JAK/STAT signaling pathway, but, instead, implicate AKT activation as a component of prolactin-induced Pim-1 transcription. Also, mitogen-activated protein kinase (MAPK) and phosphatidylinositol-3-kinase (PI-3-kinase) pathways may mediate prolactin-induced Pim-1 expression (Kumenacker et al., supra).
The human Pim-1 gene encodes a 313 amino acid serine-threonine kinase (Padma et al., Cancer Res. 51: 2486-2489 (1991); Hoover et al., J. Biol. Chem. 266: 14018-14023 (1991)) and is associated with multiple cellular functions such as proliferation, differentiation, apoptosis and tumorigenesis (Wang et al., J. Vet. Sci. 2: 167-179 (2001)). Several cellular substrates of Pim-1 have been identified, including the transcription factors cMyb (Winn et al., Cell Cycle 2: 258-262 (2003)) and NFATc1 (Rainio et al., J. Immunol. 168: 1524-7 (2002)), transcriptional co-activator of cMyb p100 (Leverson et al., Mol. Cell 2: 417-425 (1998)), phosphatases Cdc25A (Mochizuki et al., J. Biol. Chem. 274: 18659-18666 (1999)), and PTPU2S (Wang et al., J. Biol. Chem. 274: 18659-18666 (2001)), Pim-1 associated protein 1 (PAP-1) (Maita et al., Eur. J. Biochem. 267: 5168-5178 (2000)), cell-cycle inhibitor p21/WAF1 (Wang et al., Biochem. Biophys. Acta 1593: 45-55 (2002)), heterochromatin protein 1 (HP1) (Koike et al., FEBS Lett. 467: 17-21 (2000)), TRAF2/SNX6 (Ishibashi et al., FEBS Lett. 506: 33-38 (2001)) and nuclear mitotic apparatus (Bhattacharya et al., Chromosoma 111: 80-95 (2002)).
The consensus sequence for Pim-1 substrate recognition is Lys/Arg-Lys/Arg-Arg-Lys/Arg-Leu-Ser/Thr-X (SEQ ID NO:1), where X is an amino acid with a small side chain (Friedmann et al., Arch. Biochem. Biophys. 298: 594-601 (1992); Palaty et al., Biochem. Cell. Biol. 75: 153-162 (1997)). A detailed analysis of the autophosphorylation sites of Xenopus Pim-3 (previously incorrectly identified as Pim-1) has also been reported (Palaty et al., J. Biol. Chem. 272: 10514-10521 (1997)).
Due to the lack of structural information about Pim-1, the detailed mechanism of the protein is not known. Without such structural information and knowledge of the mechanism, the progress in designing drugs as specific inhibitors is impeded. Structural information on the unique features of the active site of Pim-1 would facilitate drug discovery and the treatment of cancer.